1. Field of the Invention
This invention relates to a reflection type optical disk.
2. Description of the Related Art
A reflection type optical disk has an information recording surface on which rows of pits corresponding to information signals are formed as tracks. The signals are reproduced by utilizing the phenomenon that when a reproduction or read beam spot is irradiated on a row of pits, the diffraction by the pits greatly reduces the amount of reflected light. A change in the amount of reflected light, which is obtained when the beam spot passes through the mirror portion and the target pit portion, is converted into an electric signal by a light-receiving unit, thus yielding the corresponding information signal.
For such an optical disk, the signal reproduction resolution is limited mostly by the wavelength k of the light source of the reproducing optical system and the number of apertures, NA, of the objective lens, and the spatial frequency (2NA/.lambda.)) is the cutoff spatial frequency that defines the limit of information signals. To achieve high density recording of pits on the optical disk, therefore, it is necessary to shorten the wavelength .lambda. of the light source of the reproducing optical system (e.g., a semiconductor laser) and to increase the number of apertures, NA, of the objective lens. Due to technical limitation on the improvement of the wavelength of the light source and the number of apertures of the objective lens, however, it is difficult to remarkably improve the recording density.
Under such circumstances, Japanese Patent Laid-open No. 3-292632 discloses an optical disk whose reflective film is formed of a material of which reflectance varies with temperature, such as a thermochromic material or a phase-changeable material, to thereby partially change the reflectance within the read beam spot. This structure can reduce the effective spot size of the read beam spot on the optical disk, allowing the reproduction of a spatial frequency above the cutoff spatial frequency defined by the reproducing optical system.
Since the aforementioned material changes its material structure with a change of its temperature to vary the reflectance, its response speed is generally slow and its repetition characteristic is still poor. Furthermore, it is necessary to perform an erasing operation to return the changed material structure to the initial status.